This invention pertains generally to the field of telecommunications and more particularly to repeatered carrier systems, such as pulse code modulation (PCM) systems using a transmission cable pair as the transmission medium. The repeaters for such systems are usually powered via the transmission cable pair from a central office where the terminal equipment is located. A simplex power loop is a most attractive method for providing DC power feed current to signal line repeater. For a discussion and an illustration of a simplex power loop refer to "Bipolar Repeater for PCM Signals" by J. S. Mayo, Bell System Technical Journal, Jan. 1962, pages 73-76.
A transmission cable pair (hereinafter referred to as a signal line) with exposure to lightning strikes is susceptible to induced longitudinal currents. These induced currents potentially can cause transmission impairment with the communication signals and interference with the DC power feed current supplied to the signal repeaters installed along the signal line. The effect of the induced currents upon the simplex power loop is to cause the DC power feed current in the simplex power loop to be amplitude modulated. The induced current causes the instantaneous line current, i.e., DC power feed current plus the induced current, to vary above or below some nominal level. When the instantaneous line current is above the minimum level necessary to power the repeater, the excess power is usually converted into heat primarily in the signal line by the action of the voltage control circuitry within the repeaters. However, when the instantaneous line current is below the minimum level necessary for operation of the repeater, the repeater is starved for power and fails to perform properly. Thus, it is important to control the effect of lightning induced currents on the DC power feed to the repeater.
The reduction of interference to the DC line current from lightning strikes has been regarded as unusually difficult or even impossible to treat effectively. It is common to use breakdown devices which are normally non-conductive and which suddenly are rendered fully conductive when the voltage reaches a prescribed value. However, such techniques do not necessarily provide for adequate control of the DC power feed current to the repeater during the breakdown period.